Three-phase slot antenna system



Nov., S, @949 R. a WEHNER 294%?,522

THREE-PHASE SLOT ANTENNA SYSTEM Filed Feb. '291,A 3.94@ 4 Smets-Sheet l20 210 zwwswfm" 2702600 50 40 INVENTOR ATTORNEY Nom 1949 R. s. wal-mmTHREE-PHASE SLOT ANTENNA SYSTEM 4 Sheets-Sheet 2 Filed Feb. 28, 1946 500,9417 .930 1020 .M60 1100 jf40 mm1/Mfr w Me WM L L www w j M w H En TlW-llw- ,MCM7 Mi- VQMM m M 0W M N zal? MFA/cy Innunmm l .lum M lllm... ylll-i ATTORNEY Nw@ @y w49 R. s. WEHNER www THREE-PHASE SLOT ANTENNASYSTEM Filed Feb. '28, 194e 4 Shenyang@ a i ,2 il F g 17@ 26:22, 2C-5f@fffa INVENTOR ATTO RN EY @www Nom 89 1949 R. s. WEHNER THREE-PHASE SLOTANTENNA SYSTEM 4 Sheets-Sheet 4 Filed Feb. 2S, 3.946

ATTORNEY Patented Nov. 8, 1949 2,487,622 THREE-PHASE SLOT ANTENNA SYSTEMRobert S. Wehner,

of Delaware Application February 28,

Port Jelerson, N. Y., assignor to Radio Corporation of America,

a corporation 1946, Serial N0. .650,952

12 Claims. (Cl. Z50-33) The present invention relates to short waveantennas and, more particularly, to such antennas for use on modern highspeed airplanes.

An object of the present invention is the provision oi an antenna forhigh speed airplanes which introduces no aerodynamic drag.

A further object of the present invention is the provision of an antennafor high speed airplanes which is entirely faired into the body of thefuselage or wing of the airplane.

A further object of the present invention is the provision of an ultrahigh frequency antenna having the following electrical characteristics:

l. A field pattern for horizontal polarization that is substantiallyuniform in the horizontal plane and in the hemisphere below the horizon.

2. Impedance characteristics such that the antenna may be matched With aless than 2:1 standing wave ratio to a 50 ohm line over a frequency bandat least 10% wide.

The foregoing objects and others which appear from the followingdetailed description are attained in accordance with the principles ofthe present invention by providing a triangular arrangement of threethin rectangular slots cut in the conductive skin ci the ship in eitherthe fuselage or wing and backed by resonatlng cavities. The cavities arefed in a three-phase arrangement such that horizontally polarized energyis radiated in a substantially uniform field pattern and such thatrelatively flat input impedance characteristics are attained.

The present invention will be more fully understood by reference to thefollowing detailed description which is accompanied by a drawing inWhich:

Figure l illustrates in a plan view a simple single rectangular slot andthe radiating pattern in planes parallel to the mounting surface of saidantenna, while Figure 2 is a transverse cross sectional View of theantenna of Figure l;

Figure 3 is a graph illustrating the variation in anti-resonantresistance of the antenna of Figure 1 with a variation in the placementin the feed point to the antenna.

Figure 4 is a eld strength pattern showing the distribution ofhorizontally polarized energy in the plane of the antenna of the presentinvention;

Figure 5 is a family of curves illustrating the resistance-reactancecharacteristics of a single slot and a three-phase slot antenna over aband of frequencies, while Figure 6 is a curve 4of the antennaof Figures7 and 8 illustrating the band width the present invention;

illustrate in schematic form two representative ways in which theantenna of the present invention may be energized, while Figure 9 is acurve illustrating the relationship between the standing wave ratio onthe branch lines of Figure 8 with reference to the input standing waveratio on the main transmission line;

Figure l0 is a sectional view of the antenna of the present invention,while Figure 11 is a transverse section taken along lines II, II ofFigure l0.

A simple faired-in slot antenna, suitable for use on aircraft, is shownin Figure 1. This includes a thin rectangular slot I2 cut in theconductive skin of the ship, The skin of the ship acts as a conductiveground plane and is denoted by reference letters GP. The minimum lengthL of the slot must be of the order of onehalf wavelength at theoperating frequency while the width W of the slot may be quite narrow,only 1/20 or 1/30 of a wavelength or less, so it is mechanicallypractical for use at frequencies in ultra high frequency ranges. Such aslot may be fed in a variety of ways, but one of the easiest methodsinvolves backing the slot with a rectangular resonating cavity I4(Figure 2) of the same cross section and a quarter wave in depth. Thiscavity may be fed from the coaxial cable TL, the outer conductor I6 ofwhich is connected to one lengthwise wall of the cavity at some point onits vertical center line and the inner conductor I8 of which extendsthrough cavity I4 to tie on to the opposite side wall.

Since the electric lines of force are directed across the cavity,parallel to the short sides, the radiating surface of the antenna may beregarded as the displacement current sheet across the slot aperture. Thedisplacement current sheet is likewise polarized across the slotaperture. Consequently, the field pattern in planes parallel to that inwhich the slot is cut is linearly polarized in a directon transverse tothe slot, The resultant radiation pattern, denoted by line 20 in Figurel, is roughly a figure-eight with maxima in directions parallel to theslot and with minima in directions transverse to the slot. The degree ofdirectivity is about the same as that existing in a plane containing athin half wave dipole, the difference being that in the case of a dipoleantenna, the maxima occur in directions transverse to the length of theantenna. It is apparent that a half wave slot such as shown in Figure 1is as unsuitable for uniform horizontally polarized field patterns as asimple horizontal half wave dipole would be.

Aside from the radiational limitation mentioned above, the simple slotantenna of Figure l is marked by input impedance characteristics samemanby the expression Z1Zg=(Zo/2)2 are the input impedance of the thedipole antenna respectively and Z is the lmpedance of free space.

theoretical basis for twoy outof the slot antenna input impedancecharacteristics; pedance level and the fact that it is anti-reso`l nantrather than resonant. Since Zo, the imabove resonance, the reverse mustbe true of a The input impedance character-` istics of a slot 25centimeters long, 1 centimeter wide, backed by a cavity of the samecross secdeep, were measured for several positions of the feedtransmission line TL over a range of frequencies extending from 900 to1150 megacycles. One set of the resultant impedance data is graphicallyshown in Figure 5 wherein curve 25 illustrates the variation inresistance with a Variation in the frequency while curve 26 illustratesthe variation in reactance. These curves reveal the single slot antennato be sharply anti-resonant with a high impedance level. Data run atother positions of the feed point, that is, at the other values of theratio d/D, where d is the distance of the cn- The chief effect of feedposif tion upon impedance characteristics is in terms the region overwhich measurements were actu- This or other standard low impedance linesover only relatively narrow frequency bands. For example,

mission line by means of a conventional quarterwave geometric meantransformer yields only 31.4% band width with less than a 2:1 standingwave ratio on a 50 ohm line. This is indicated by curve 30 of Figure 6.It might be expected thatl the simple slot antenna reactive relative tothe impedance level; that is', it has a much higher value of QV andthere'- fore a smaller intrinsic bandr width than would be obtained bymatching a high impedance slot down to a low level. Greater band widthsmight possibly be realized by use of'complicated matching sections butit is believed that any of the type shown in ratio on a standard 50 ohmcable.

Now consider a delta shaped array of antenna slots such as thoseindicated in the diagram at the center of Figure 4 wherein the slots areidentied by reference characters 4l, 42 and 43. This arrangement, withthe displacement-cur'- rent sheets across the slot apertures representedby suitably averaged quarter wave long, the iield in the plane of thearray may be shown to be given by the following expression:

where 1 @wat [1 -l-cos2 0-2 cos 0 sin (90 cos 6)] (2) and nre. 5 slotantennas, may be matched to 50v Ohm Tv muthal angle a, and

where RPE2 signifies Real Part of E2 IPEZ sianiiies Imaginary Part ofE2, etc.

Equation 1 gives the relative iield strength in the plane of the arrayas a function of the. azlis plotted in Figure 4 as. curve 45. It will benoted that curve 45 is very nearly circular, exhibiting an even greaterdegree of circular symmetry than is yielded by a turnstile array; thatis, by two crossed dipoles fed in phase quadrature. Such arrays oflinear radiators have been commonly used heretofore in applicationscalling for uniform distribution of horizontally polarized radiation,but the application of this principle to slots is believed to be new. Itwill be noted that in the plane of the slots the polarization is linearbut rotates with the carrier frequency. In the direction normal to theplane the polarization is circular, while in intermediate directions thepolarization is elliptical.

The neld pattern shown in Figure 4 is not affected by moderatedeviations from resonant frequency. The three-phase antenna yieldssubstantially the same pattern over the entire range over which itsinput impedance characteristics are such that it may be matched to astandard transmission line, that is, input impedance rather than patterncharacteristics will limit the useful bandwidth of the antenna.

It is readily evident that if n antennas of equal input impedance Za areto be fed with currents of equal magnitude an 'ri-phase relationshipfrom a matched main transmission line of characteristic impedance Zo,the individual input impedances Za must equal nZo and the individualantennas must be fed by the equivalent of branch lines of characteristicimpedance nZo, each successive branch line differing in length from thepreceding by degrees. This result may be attained in two slightlydifferent mechanical arrangements. The two arrangements areschematically illustrated in Figures 7 and 8.

In Figure 7, for example, the main transmission line TL, havingcharacteristic impedance Z0, is directly connected across the firstantenna identified by reference numeral 50, having an input impedance of3Zo. In shunt across this antenna and across transmission line TL isconnected a second section of transmission line TL1 having an electricallength of 120 and having a characteristic impedance equal to 1.520. Thissection of transmission line TL1 leads to the second antenna likewisehaving an input impedance of 32o and to a second section of transmissionline TLz, likewise having an electrical length of 120 and having acharacteristic impedance of 3Z0. Transmission line TL2 is terminated byits connection to the third antenna 52 having an input impedance of 3Zo.

The alternative arrangement shown in Figure 8 employs three physicallyseparate and distinct transmission line sections TLi, "ILz and TLs.These three sections of transmission lines are all connected in parallelat one end and at the other ends are directly connected to antennas 50,5l and 52. As before, the antennas each have an impedance 3Zo. Each ofthe transmission lines has a characteristic impedance equal to 32.0 andthey differ in length by 120 electrical degrees. In the event that theindividual antennas 5f), 5l and 52 are not perfectly matched to theindividual branch lines; which will be the case if the system is tooperate over an appreciable range of frequencies; perfect n-phasefeeding will no longer exist and this may be expected to have at leastsome adverse effect both on the field pattern and on the input impedanceof the array.

. possibility of interaction However, this effect is much smaller thanmight be expected. In the case of three identical antennas fed inthree-phase relationship, it can be shown that if 7c is the magnitude ofthe reflection coefficient due to mismatch on the individual branchlines, and if K is the magnitude of the reflection coefficient due tothe mismatch on the main feed line, then these two quantities arerelated by the expression K==7c3. Since the magnitude of the reflectioncoefficient is by definition less than unity it is evident thatthree-phase feeding has a pronounced broad banding effect. In terms ofthe standing wave ratio, more commonly used as a measure of mismatch,the aboveexpression is equivalent to s (S3-P38) (BSZ-t1) where S is thestanding wave ratio on the main line and s is the standing wave ratio onthe branch lines. This expression is plotted in Fig-Y ure 9 andindicates that the standing wave ratioon the branch lines, plotted asabscissae, can become quite high before the standing wave ratio on themain line, plotted as ordinates, becomes appreciably greater than unity.

While the main line reflection coefficient decreases as the cube of thatexisting on the branch lines only at or near the resonant frequency,that is, at that frequency at which the length of the branch lines areproper, the three-phase system has compensating tendencies over anappreciable range of frequencies to either side of resonance. That suchcompensating tendencies are appreciable even with a system involvingindividual antennas as inherently narrow band as the simple slot antennaof figure (Z) is demonstrated by curves 54 and 55 of Figure 5. Thesecurves were calculated from measured input impedance data for a singleohm slot antenna input impedance, combined in a three-phase arrangement.It is evident by comparing curves 54 and 55 with 25 and 26 which relateto a single slot antenna that the input impedance of the threephasearray is much flatter than that of the component antennas. The bandwidth of a threephase array fed directly without an external matchingsection is 15.2% with less than a 2:1 standing wave ratio on a 50 ohmline as is lshown by curve 56 of Figure 6, as compared with the 3.4% fora single slot and matching section as shown by curve 30.

In the foregoing example of the broad banding effect of three-phase feedon slot antennas, the between the individual slots has been ignored.This'is not a serious objection to the arrangement, however, since it isevident, from considerations of symmetry and the fact that all threeslots are identical, that the mutual impedances between any pair ofslots in the array is the same as that existing between' any other pairof slots. Furthermore, the input impedance of any one slot in the arrayis equal to the difference between the self-impedance of that slot, thatis, its input impedance when mounted by itself in a large ground plane,and the mutual impedance between any pair of slots. So regardless of themagnitude of the mutual impedances between the slots their inputimpedanoes are identical.

Of course, it is not sufficient that the individual input impedances ofthe component antennas be identical. rIhey must also be equal to threetimes the characteristic impedance of the main transmission line.Howeven this makesv little difference as far as the practical use of theantenna is concerned si-nce any input impedance impedance between theslots may be compensated for merely the position Furtherplane beingparallel to the of the feeds of the individual slots. The individualslots are backed up by cavities 50, 62 and 64 having the same transversedimensions as the slots and approximately a quarter wave in depth. Thecavity 64 carries on one side wall an input cable connector TLC having athreaded outer acteristicl impedance of 70 ohms. The said innerconductor eventually passes through cavity 52 having an outer shell 'l5and an inner conductor T6. An adjustableshorting plug T8 is providedconstructed of a low loss dielectric such as polyethylene orpolystyrene. At the point Where conductor 10 passes into slot 62 aninner conductor 12 branches off and being surrounded by outer shell `|3forms a feed line for cavity 50. Said feed line like that formed byconductors 'I0 and 7| has an electrical length of 120 degrees. Thediameters of conductors 12, I3 are so proportioned as to provide acharacteristic impedancel of 140 ohms. Conductor '|2, after passingthrough cavity Si), is terminated in an end stub 82, the conductor |2lbeing connected electrically to theI end of the outer shell 82 of stub82.

As shown in Figure 11 the feed plane of the antenna is located farenough above the base of cavities 60, '62 and 64 array that thev inputimpedances of the separate slots, allowing for the eiect of interactionare each equal to 140 ohms, although any gure between and |65 would besatisfactory for a.- delta slot system fed from. a I ohm coaxial line.The system is thus approxithree feed positions may be mately equivalentto three` 140 ohm. impedancesl the overall input impedance isapproximately one third of 140 or 47 ohms, close ing radiation eld isset up.

If desired, the input connector TLC can `be located on the inner wall ofcavity 64 rather than on the outer' wall as shown in Figure 10. Theouter wall is preferable, from the standpointl of convenience in makingconnections to the transmitter. Since cavity 64 is thus fed in adifferent manner from cavities 60 and 62 some slight discontinuityeffect will be noted.

system shown in Figure 10, the restrictionis notl severe. Ifair-dielectric cavities and lines are used, the maximumy slot length isapproximately 2/3 of the operating mid-band wavelength. Sincey deviatefrom resonance by at least 24` to either side before the system becomesinoperative dueto insufficient slot length. A potential band Width of atleast 48% is considerably larger than that over which the impedancecharacteristics willbe satisfactory.

The cavities 69 and 52 are preferably closed by thin conductive window(Figure 11) of polystyrene or other low loss' dielectric material'.v Theparticular manner of' securing the closure where-l by the cavities arerendered The anges may easily be screwed or cemented on to the sidewalls of the cavity.

While I have illustrated a particular embodiment of the presentinvention, it

ing that a rotating eld is radiated.

2. An antenna system including a numberl of elongated narrow slots in aconductive sheet, said in a delta formation, a conof high frequencyenergy and a multi-branch transmission line connecting said source toeach of said cavities, the lengths of the connections from. said sourcevto said cavities so differing that a rotating eld is radiated.

3. A n antenna system including a number of elongated narrow slots in aconductive. sheet.

said slots `being arranged in a delta formation, a conductive walledcavity back of each slot, a source of high frequency energy and amultibranch transmission line connecting said source to each of saidcavities, the lengths of the connections from said source to saidcavities so differing that a rotating field is radiated, the height ofthe point of entry of said connections into said cav ities being sochosen that the impedance presented at said connections bysaid cavitiesis equal to the impedance of said connections.

4. An antenna system including a number N of elongated narrow slots in aconductive sheet, said slots being arranged in a regular geometricpattern, a conductive walled cavity back of each slot, a source of highfrequency energy and a multibranch transmission line having a primarysection of Zu ohms characteristic impedance and branch sectionsconnecting said source to each of said cavities, the lengths of theconnections from said source to successively energized cavitiesdiffering by is N degrees whereby a rotating field is radiated, theheight of the point of entry of said connections into said cavitiesbeing so chosen that the impedance presented at said connections by eachof said cavities is N times the characteristic impedance of the saidprimary section of transmission line, or equal to the characteristicimpedances NZo ohms of said branch sections.

5. An antenna system including a number of elongated narrow slots in aconductive sheet, said slots beingarranged in delta formation, aconductive walled cavity back of each slot, a coaxial transmission linefrom high frequency energy transducer means, the inner conductor of saidline being coupled to each of said cavities in turn to energize thesame, the length of line between successively energized cavities beingsuch that a rotating field is radiated.

6. An antenna system including a number of elongated narrow slots in aconductive sheet, said slots being arranged in delta formation, aconductive walled cavity back of each slot, a coaxial transmission linefrom high frequency energy transducer means, the inner conductor of saidline being coupled to each of said cavities in turn to energize thesame, the length of line between successively energized cavities beingsuch that a rotating field is radiated and the characteristic impedanceof said transmission line so differing between successively energizedcavities that the impedance of the transmission line at any point isequal to the resultant parallel impedance of the first succeeding cavityand the succeeding portions of the system remote from said transducermeans.

'7. An antenna system including a number N of elongated narrow slots ina conductive sheet, said slots being arranged in a regular geometricpattern, a conductive walled cavity back of each slot, a coaxialtransmission line from a high frequency energy transducer means, theinner conductor oi said line being coupled to each of said cavities inturn to energize the same, the length of the line between successivelyenergized cavities being in N electrical degrees and the characteristicimpedance of said transmission line so differing between successivelyenergized cavities that the impedance 10 the resultant parallelimpedance of the first suc-i ceeding cavity and the succeeding portionsof said system remote from said transducer means.

8. An antenna system including a number N of elongated narrow slots in aconductive sheet, said slots being arranged in a regular geometricpattern, a conductive walled cavity back of each slot, a coaxialtransmission line from a high frequency energy transducer means, theinner con ductor of said line being coupled to each of said cavities inturn to energize the same, the height of said line in said cavitiesbeing such as to present to said line an impedance N times that of saidline, the length of the line between successively energized cavitiesbeing electrical degrees and the characteristic impedance of saidtransmission line so differing between successively energized cavitiesthat the impedance of the transmission line at any point is equal to theresultant parallel impedance of the first succeeding cavity and thesucceeding portions of said system remote from said transducer means.

9. An antenna system including a number N of elongated narrow slots in aconductive sheet, said slots being arranged in a delta formation, aconductive walled cavity back of each slot, a source of high frequencyenergy and a multi-branch transmission line connecting said source toeach of said cavities to energize the same, the lengths of theconnections from said source to successively energized cavitiesdiffering by in N electrical degrees whereby a rotating field isradiated.

10. An antenna system including a number N of elongated narrow slots ina conductive sheet, said slots being arranged in delta formation, aconductive walled cavity back of each slot, a coaxial transmission linefrom high frequency energy transducer means, the inner conductor of saidline being coupled to each of said cavities in turn to energize thesame, the length of line between successively energized cavities beingsuch that a rotating field is radiated, said transmission line beingterminated in an adjustable shortcircuited tuning stub whereby the feedto said cavities may be equalized.

11, An antenna system including three elongated narrow slots in aconductive sheet, said slots being arranged in delta formation, aconductive walled cavity back of each slot, a coaxial transmission linefrom high frequency energy transducer means to energize said cavities,the inner conductor of said line passing through the first and last ofsaid cavities and having a branch connection passing through the secondof said cavities, the length of line between successively energizedcavities being such that a rotating field is radiated and thecharacteristic impedance of said transmission line so differing betweensuccessively energized cavities that the impedance of the transmissionline at any point is eoual to the resultant parallel impedance of thefirst succeeding cavity and the succeeding portions of the system in thedirection of said last cavity, said second and last cavities beingprovided with short-circuited tuning stubs connected to said linewhereby the feed to said cavities may be equalized.

l2. An antenna system including a number of elongated narrow slots in aconductive sheet, said O the trarlSmSSlOr! lille at any l 011t iS equalt0 75 slots being arranged in a polygonal formation in 'said's'he'et aoondiictiVewalledv cavit'ybck ofeavcliV Y slot, a transmission linevfrom high frequency toV energize thev same, thev lengthv of line betweensuccessively energized cavitiesk being such that a rotating eld isradiated'.

ROBERT S. WEHNER;

REFERENCES CITED' The following references are of record in the lef ofthisy patent:

112 UNITED STATES' PATENTS Number

